In current plastic cards typified by, for example, credit cards and cash cards, cards with hologram have come to be widely used mainly from the viewpoint of the prevention of replication and impartation of designs. Because the hologram has the advantage that not only it has excellent three-dimensional expression and beauty but also its replication is difficult, it is frequently used in security applications and the like.
Holograms are based on the principle that two lights (object light and reference light) having the same wavelength are made to interfere with each other to record the wave-front of the object light as interference fringe in a photosensitive material, and when light having the same wavelength as the reference light used in the recording of the interference fringe is applied, the recorded interference fringe causes a diffraction phenomenon, enabling the reproduction of the same wave-front as that of the original object light.
Holograms as described above may be classified into a few types by the recording form of the interference fringe. Typically, these holograms may be classified into a surface relief type hologram and volume hologram. Here, the surface relief type hologram is the type of hologram in which a fine concave and convex pattern is formed on the surface of the hologram layer to thereby record a hologram. On the other hand, the volume hologram is the type of hologram in which interference fringes produced by the interference of light are three-dimensionally written as fringes differing in refractive index in the direction of the thickness to thereby record a hologram. Because the volume hologram is the type of hologram in which a hologram image is recorded by a difference in the refractive index of material, it is more difficult to replicate a hologram image than in the case of a relief type hologram. The volume hologram is therefore expected to be useful in applications for means of preventing forgeries of marketable securities and cards.
Various methods are known corresponding to an object to which a hologram is provided as a method of providing a hologram to marketable securities and cards in the case of using a hologram as means for the impartation of designs and prevention of forgeries. Though a method in which a slit-like hologram is webbed and a method in which a hologram is embedded in a medium in such a manner as to be visible from the outside are known, a method in which a hologram is attached to a predetermined position is generally used. Among these methods, a more simplified method has come to be widely used in which a hologram layer is transferred from a hologram transfer foil with a hologram formed on an optional base material to thereby apply the hologram layer to a predetermined position.
Here, in volume holograms, a plurality of materials differing in refractive index is generally used and, usually, photopolymerizable materials which can be polymerized by applying specific light are used. For this, it is known that the hologram layer in which the volume hologram has been recorded tends to be increased in mechanical strength. Further, because the volume hologram is characterized by the feature that a hologram image is recorded by three-dimensional arrangement of difference in refractive index, the thickness of the layer where the hologram is formed tends to be larger than that of the above relief type hologram. It has been therefore pointed out that the volume hologram layer is inferior in foil cutting characteristics and it is therefore difficult to use the method of transferring a hologram layer by using the hologram layer transfer foil.
In such a situation, Patent Literature 1 discloses an example in which the rupture elongation and rupture strength of the hologram layer in which a volume hologram is recorded are adjusted to predetermined values to thereby also enable the volume hologram to use the aforementioned transfer method using the hologram layer transfer foil. However, there is the problem that it is difficult to attain the compatibility between clear recording of a hologram image and adjustment to a predetermined elongation at breakage and rupture strength from, for example, the reason that usable materials used to record a clear hologram image are limited.
Further, the impartation of designs and prevention of forgeries which are achieved using a hologram are desired to be more improved in versatility in view of its usefulness. Along with this, a method has been developed in which the hologram transfer foil is made into a lengthy form to continuously transfer a volume hologram thereby enabling the volume hologram to be continuously disposed on a predetermined position in an industrial production process. However, this requires partial transfer of the volume hologram formed on the transfer foil, with the result that higher foil cutting characteristics are required.
In light of the above problem, Patent Literature 2 discloses technologies in which a notch is formed in the hologram layer to improve the foil cutting characteristics of a hologram layer. Although the foil cutting characteristics are improved by the technologies in which a notch is formed, the technologies had the problem that, for example, the notch remains in the product obtained after the transfer operation, which spoils the beauties of the product and have come to further involve a new problem concerning productivities such as costs and complicated production process. Accordingly, it is desired to develop a transfer foil which has foil cutting characteristics improved to the extent that partial transfer can be attained without any problem even if a notch or the like is not formed.